A chemically dominant polish mechanism ensures high selectivity to the surrounding isolation dielectrics, allowing the process to effectively halt once the field tungsten is cleared .
In depth
In advanced Back-Side Illuminated (BSI) CMOS Image Sensors, inter-pixel optical crosstalk is mitigated by constr
ucting deep metallic light shield grids between adjacent photodiodes (Engineering Practice). Following the deposition of the mid vertical grid, where conformal tungsten completely fills the high-aspect-ratio trench structures, the resulting thick metallic overburden must be entirely removed (Engineering Practice). The Tungsten Chemical Mechanical Polishing (W CMP) step serves to globally planarize the wafer surface and isolate the individual grid lines by stripping the field tungsten down to the underlying dielectric . This highly controlled planarization is essential for establishing a flat, defect-free topography required for subsequent photolithography and the deposition of the upper vertical grid barriers . The fundamental material removal mechanism in W CMP relies on a synergistic "chemical oxidation—mechanical removal" continuous cycle . Chemically, oxidizers in the slurry, predominantly hydrogen peroxide (H2O2) often accelerated by catalysts like iron nitrate (Fe(NO3)3), react with the exposed bulk tungsten to form a softer, hydrated passivating oxide layer such as WOx . Mechanically, nanoscale abrasive particles suspended in the slurry are pressed against the wafer by the asperities of a polymeric polishing pad, sliding and rolling across the surface to physically shear away this passivated oxide . This chemical modification severely reduces the "dynamic hardness" of the metal surface, shifting the mechanical removal from bulk metallic ploughing to a nanoscale selective plastic deformation and micro-cutting of the weakened oxide layer . Once the mechanical abrasion clears the local oxide, fresh underlying tungsten is immediately exposed to the slurry, re-initiating the rapid oxidation phase and sustaining the removal rate . The optimization of W CMP requires a delicate balance of physical and chemical input parameters, rooted heavily in contact mechanics and reaction kinetics . While the baseline mechanical material removal rate roughly follows the Preston equation—scaling proportionally with applied down-pressure and relative rotational velocity—it is heavily modulated by the pad surface properties and interface temperatures . Frictional heating during polishing accelerates the thermochemical oxidation reactions at the pad-wafer interface, making precise platen coolant temperature control a critical factor to prevent over-oxidation and maintain a stable removal rate . Furthermore, the choice of polishing pad directly impacts the spatial distribution of mechanical stress; utilizing a porous pad with discrete, localized segmentations helps to suppress the lateral propagation of deformation under load, thereby vastly improving within-wafer (WIW) planarization uniformity and edge profile control . Simultaneously, the slurry's acidic pH and oxidizer concentration must be strictly bounded, because over-oxidation without sufficient mechanical clearing generates thick, porous films that degrade removal efficiency and surface morphology . For nanoscale-node BSI architectures, the dense array of fine-pitch vertical grid trenches dictates that purely mechanical polishing would cause unacceptable dishing and structural damage . A chemically dominant polish mechanism ensures high selectivity to the surrounding isolation dielectrics, allowing the process to effectively halt once the field tungsten is cleared . Immediately following this precise planarization, the wafer transitions to a post-CMP cleaning module utilizing a weakly alkaline solution enriched with organic amines . These amines act as Lewis bases, neutralizing residual acidic oxidizers and complexing with high-energy sites on the freshly polished tungsten to form a protective carbon-enriched buffer layer, ultimately preventing galvanic corrosion and minimizing interface defect density for the subsequent barrier deposition .
Risks & Challenges
[High] Severe Tungsten Dishing and Plug Recess: If the chemical reaction rate vastly outpaces mechanical removal, excessive oxidizers (e.g. , H2O2) rapidly convert the grid trench tungsten into thick, soft WOx layers . When coupled with a polishing pad lacking adequate localized rigidity or segmentation, long-range elastic deformation forces the pad into the trench cavities, mechanically clearing the oxide and resulting in deep local recesses .
[Medium] Within-Wafer (WIW) Polishing Non-Uniformity: The thermochemical oxidation of tungsten is highly temperature-dependent, driven by the frictional heat generated at the abrasive-wafer interface . Without optimized platen coolant flow, significant radial temperature gradients manifest across the polishing pad, causing spatially variable chemical reaction rates that lead to incomplete clearing (under-polish) or localized erosion (over-polish) .
[Medium] Post-CMP Tungsten Corrosion and Interface Degradation: The pristine tungsten surface exposed at the end of the CMP step contains high-energy unsaturated bonds and residual acidic oxidizers . Failing to immediately quench these reactive sites with a weakly alkaline post-CMP clean allows uncontrolled native oxide regrowth or localized galvanic corrosion, permanently degrading the electrical and structural integrity of the subsequent metal interfaces .
[Low] Deep Nanoscale Scratching and Defect Generation: Although the tungsten surface is chemically softened to reduce its local yield threshold, the agglomeration of slurry abrasive particles can create abnormally large stress concentrators . These oversized aggregates induce excessive Hertzian contact pressures during sliding friction, penetrating the passivated WOx layer and causing severe plastic ploughing and micro-cutting into the bulk metal and surrounding dielectrics .